Capacitive heating apparatus for drying an object having a dielectric value and geometrical dimensions which vary as such object is dried



3,396,474 CT HAVING WHICH Aug. 13, 1968 CAPACITIVE HEATING A ARATUS FOR DRYING AN OB A DIELECTRIC VALUE AND GEOMETRICAL DIMENSI VARY AS SUCH OBJECT IS DRIED Filed Jan. 9.

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United States Patent O 3,396,474 CAPACITIVE HEATING APPARATUS FOR DRY- ING AN UBJECT HAVING A DIELECTRIC VALUE AND GEMETRICAL DIMENSIONS WHICH VARY AS SUCH BJECT IS DRIED Emil Walther, Erlangen, Klaus Schmidt, Erlangen-Bruck,

and Hans-Christian Grassman, Erlangen, Germany, assignors to Siemens Aktiengesellschaft, Berlin and' Munich, Germany, a corporation of Germany Filed Jan. 9, 1967, Ser. No. 608,077 12 Claims. (Cl. 34-1) ABSTRACT OF THE DISCLOSURE Capacitive heating apparatus for drying an object having a dielectric value and geometrical dimensions which vary as the object is dried includes a pair of spaced electrodes which provide a capacitive heating field. First and second spaced auxiliary electrodes of different dimensions are positioned in the capacitive field between the electrodes. Each of the auxiliary electrodes has an inner surface facing the other of the auxiliary electrodes. First and second heat insulating layers having poor dielectric loss characteristics are each afiixed to the inner surface of a corresponding one of the auxiliary electrodes. The object is positioned between the first and second heat insulating layers.

Disclosure of the invention The present invention relates to heating apparatus for drying an object having a dielectric value and geometrical dimensions which vary as the object is dried. More particularly, the invention relates to capacitive heating apparatus for drying an object having a dielectric value and geometrical dimensions which vary as the object is dried.

In known devices for the capacitive heat processing of thermoplastic objects having a very poor dielectric loss factor, one or more discs or foils of a material having a relatively high dielectric loss factor are alloyed in at least one side between the electrodes and the object to be processed. The discs or foils are not only required to adequately heat the object having a poor dielectric loss factor, but are also required to prevent a dissipation of heat from its outer layers.

In known devices for the capacitive drying of lumber and pulp or cellulose, material having a high dielectric loss factor is positioned between the object to be processed and the electrodes or plates of the capacitor. The material is heated considerably in the capacitor field, and thus prevents water vapor dissipated from the object from condensing on the capacitor plates or electrodes and thereby prevents the wetting of the object in the area of the material. In known devices for the capacitive drying of grinding discs, auxiliary electrodes are provided at one or both opposite surfaces of the grinding disc. The auxiliary electrodes divert spark-overs or arcs caused by variations in field intensity, and thus prevent localized burning of the grinding disc.

A disadvantage of the known capacitive heating devices is that the auxiliary electrodes cannot be utilized to provide a homogeneous eld distribution, so that the drying of the object to be dried is non-uniform, when said object has a dielectric value and geometrical dimensions which vary as said object is dried and has a dimension in the axial direction perpendicular to the electrodes equal to or longer than its thickness or diameter. Another disadvantage is that the cover layers of the auxiliary electrodes comprise material which has a high dielectric loss and which is heated to a higher temperature in the capacitive high frequency field than the object to be dried. This may damage or burn the object to be dried in the areas of the auxiliary electrodes.

The principal object of the present invention is to provide a new and improved capacitive heating apparatus for drying an object having a dielectric value and geometrical dimensions which vary as such object is dried. The apparatus of the prevent invention overcomes the disadvantages ofrthe known capacitive heating devices and provides uniform heating of an object having a dielectric value and geometrical dimensions which vary as such object is dried. The apparatus 0f the invention is of simple structure and operates with eiciency, effectiveness and reliability. The apparatus of the present invention avoids the ironing effect in textiles, which is the overheating of the textile object at its contact areas with heat insulating layers.

In accordance with the present invention, apparatus for drying an object having a dielectric value and geometrical dimensions which vary as the object is dried, comprises capacitive heating means including a pair of spaced electrodes for providing a capacitive heating field between the electrodes. First and second spaced auxiliary electrodes of different dimensions are positioned in the capacitive heating field between the electrodes. Each of the auxiliary electrodes has an inner surface facing the other of the auxiliary electrodes. First and second heat insulating layers having poor dielectric loss characteristics are each affixed to the inner surface of a corresponding one of the auxiliary electrodes. The object is positioned between the first and second heat insulating layers. The heat insulating layers comprise material which is permeable to gas and which has a dielectric loss coeicient having a maximum value of 6 102. Each of the heat insulating layers axially centers the object in the capacitive heating field.

Each of the auxiliary electrodes is a frustum having a base having a determined surface area. The inner surface of each of the auxiliary electrodes is a section parallel to the base and having a surface area smaller than the determined surface area. A substantially cylindrical aperture is coaxially formed through each of the frustums to provide an inner surface of substantially annular configuration. Each of the frustums has an altitude and a slant height; the first frustum having a slant height at a iir'st angle with the corresponding altitude and the second frustum having a slant height at a second angle with the corresponding altitude which is different from the first angle. The first frustum supports the object. The second frustum rests on the object and the first angle is greater than the second angle. The altitude of the second frustum is approximately twice the altitude of the first frustum.

In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawing, wherein:

FIG. l is a schematic diagram, in side view, of capacitive heating apparatus ofthe prior art; and

FIG. 2 is a schematic diagram, in side view, of an embodiment of the capacitive heating apparatus of the present invention for drying an object having a dielectric value and geometrical dimensions which vary as such object is dried.

In the capacitive heating apparatus of the prior art of FIG. l and in the capacitive heating apparatus of the present invention of FIG. 2, an object 1 to be dried is positioned in a capacitive heating field provided by a high frequency generator 2 between an electrode 3 connected to said generator and an electrode 4 spaced from the electrode 3. The electrode 4 is at ground potential and is of known configuration such as, for example, a driven metal conveyor belt. Capacitive heating is undertaken in a range of 1 to 100 megacycles per second, and the generator 2 operates at a frequency of 15 lmegacycles per second.

The object 1 to be dried has a dielectric value and geometrical dimensions which vary as said object is dried. The object 1 may comprise, for example, a waterladen rayon cake such as,v for example, disclose-d in United States Patent No. 2,325,652. The rayon cake 1 is plastic or rayon yarn wound on a spool or skein which has been freshly sprayed. A substantially cylindrical aperture 5 is coaxially formed through the rayon cake 1.

In FIG. 1, the object 1 to be dried is positioned on a base of insulating material 6. The insulating material 6 may comprise, for example, a wood composition panel. When the object 1 is heated by the apparatus of FIG. 1, said object is dried considerably less at its opposite surfaces facing the electrode 3 and the insulating material 6, respectively, than at its central areas. Experimentation has shown that such uneven drying is due more to a field distortion, which causes inhomogeneity of the drying, than to recondensation of vapor on the surfaces of the'objectl facing the electrodes 3 and 4.

The apparatus of the present invention, as shown in FIG. 2, compensates for the field distortion and permits uniform heating, and, therefore, uniform drying of the object 1. In FIG. 2, a first auxiliary electrode 8 is positioned on the electrode 4. A second auxiliary electrode 7 is spaced from the first auxiliary electrode 8. The first and second auxiliary electrodes 8 and 7 are thus positioned in the capacitive heating field between the electrodes 3 and 4. The first and second auxiliary electrodes 8 and 7 are of different dimensions. Each of the first and second auxiliary electrodes 8 and 7 has an inner surface facing the other of the auxiliary electrodes. The auxiliary electrodes 8 and 7 are coaxially positioned.

A first heat insulating layer 10, having a poor dielectric loss characteristic, is affixed to the inner surface of the first auxiliary electrode 8 by any suitable means such as, for example, adhesive. A second heat insulating layer 9, having a poor dielectric loss characteristic, is affixed to the inner surface of the second auxiliary electrode 7 by any suitable means such as, for example, adhesive. The object 1 to be dried is positioned between the first and second heat insulating layers and 9, with its surface facing the electrode 3 in contact with the second insulating layer 9 and with its surface facing the electrode 4 in contact with the first insulating layer 10.

The insulating layers 9 and 10 comprise material which is permeable to gas in order to permit the escape of vapor. The material of the insulating layers has a dielectric constant 1.5ere16 and a dielectric loss factor tg100 104- Fiberglass having a thickness of several millimeters is a preferred insulating layer material. The maximum dielectric loss coefficient of the material of the insulating layers has a maximum value of 6X10-2. The material of the insulating layers 9 and 10 must have a low dielectric loss factor, because if it has a high dielectric loss factor, the heat-sensitive woven fibers of the surfaces of the rayon cake or other object 1 facing said insulating layers would burn, due to the ironing effect, or would at least undergo a change in structure which during the subsequent drying would result in an uneven application of the dye.

The heat `storing characteristic of the material of the first and second insulating layers 10 and 9 prevents the formation of condensation at the front surfaces on the surfaces of the object 1 facing said insulating layers, during the cooling of said object. To assist in the prevention of condensation, the peripheral edges of the insulating layers are extended beyond the object 1 by providing them with a diameter which is greater than that of said object. This prevents the formation of condensation in the edge areas of the object 1 during cooling. The ratio of the diameter of the object l to that of the insulating layers 9 and 10 is preferably lzl.

Since the altitude H1 of the object l is greater than the diameter D1 of said object, it is preferable to provide an increased lateral constriction of the field lines at the beginning of the heating process in the prior art apparatus of FIG. l. This causes non-uniform heating, and hence non-uniform drying, of the object 1, as hereinbefore indicate-d. During the drying process, the open weaving of the rayon cake 1 becomes more tightly packed, so that a rayon cake having a considerable amount of water in it initially may vary up to about 50% in altitude when the heating process is terminated. The configuration of the auxiliary electrodes 3 and 7 controls the field lines in a manner whereby the object 1 is heated uniformly over its entire cross section and said object is thus uniformly dried throughout its entire altitude.

Each of the first and second auxiliary electrodes S and 7 is a frustum of, for example, pyramidal, polygonal or conical configuration having a base having a determined surface area. The frustums provide uniformity with the electric field intensity which dries the object 1. The base of the first auxiliary electrode y8 faces and rests on the electrode 4 and the base of the second auxiliary electrode 7 faces the electrode 3. The first frustum 8 has an inner surface, to which the first insulating layer 10 is afiixed, which is a section parallel to the base thereof and having a surface area smaller than the determined surface area of said base. The second frustum 7 has an inner surface, to which the second insulating layer 9 is affixed, which is a section parallel to the base thereof and having a surface area smaller than the determined surface area of said base.

A substantially cylindrical aperture is coaxially formed through each of the frustums 8 and 7, so that each of the inner surface and the base of each of said frustums is -of substantially annular configuration. The annular surfaces vprovide for a better conduction of vapor from the object 1. The auxiliary electrodes 8 and 7 may be perforated to provide for better vapor conduction from the object 1. Each of the frustums 8 `and 7 has an altitude, which is Coaxial with the other, and a slant height. The altitude H7 of the second frustum 7 is approximately twice the altitude H8 of the first frustum 8. The slant height of the first frustum 8 is at a first angle with its altitude which is greater than a second angle formed by the slant height of the second frustum 7 with its altitude. Each of the first and second angles is between 30 and 60.

The ratio of the altitude H7 to the altitude H8 is preferably 2: 1, as hereinbefore mentioned, when the ratio of the altitude H1 to the diameter D1 of the object 1 is 1:1 at the beginning of the drying process.

In order to permit a production line operation, the auxiliary electrodes 7 and 8 and/ or the insulating layers 9 and 10 are preferably provided with axial centering devices for axially centering the object 1 in the capacitive heating eld. The centering devices comprise material having poor dielectric losses.

The capacitive heating apparatus of the present invention is, of course, not limited to the drying of water-laden rayon cakes, but is advantageously utilized whenever a heat-sensitive object is to be uniformly heated and dried.

While the invention has been described by means of a specific example and in a specific embodiment, we do not wish to be limited thereto, `for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

We claim:

1. Apparatus for drying an object having a dielectric value and geometrical dimensions which vary as said object is dried, comprising capacitive heating means including a pair of spaced electrodes for providing a capacitive heating field between said electrodes;

first and second spaced auxiliary electrodes of difierent dimensions positioned in said capacitive heating field between said electrodes, each of said auxiliary electrodes having an inner surface facing the other of said auxiliary electrodes; and

first and sec-ond heat insulating layers having poor dielectric loss characteristics each affixed to the inner surface of a corresponding one of said auxiliary electrodes, said object being positioned between said first and second heat insulating layers.

2. Apparatus as claimed in claim 1, wherein said heat insulating layers comprise material which is permeable to gas.

3. Apparatus as claimed in claim 1, wherein said heat insulating layers comprise material having a dielectric loss coefficient having a maximum value -of 6X10-2.

4. Apparatus as claimed in claim 1, wherein each of said heat insulating layers axially centers said object in said capacitive heating field.

5. Apparatus as claimed in claim 1, wherein each of said auxiliary electrodes is a frustum.

6. Apparatus as claimed in claim 1, wherein each of said auxiliary electrodes is a frustum having a base having a determined surface area and the inner surface of each of said auxiliary electrodes is a section parallel to said base and having a surface area smaller than said determined surface area.

7. Apparatus as claimed in claim 1, wherein each of said auxiliary electrodes is a frustum having a base having a determined surface area, the inner surface of each of said auxiliary electrodes is a section parallel to said base and having a surface area smaller than said determined surface area, and a substantially cylindrical aperture is coaxially formed through each of said frustums to provide an inner surface of substantially annular configuration.

8. Apparatus as claimed in claim 7, wherein each of said frustums has an yaltitude and a slant height, the first frustum having a slant height at a first angle with the corresponding altitude andthe second frustum having a slant height at a second angle with the corresponding altitude which is different from said first angle.

9. Apparatus as claimed in claim 7, wherein the altitude of the first frustum is different from the altitude of the second frustum.

10. Apparatus as claimed in claim 8, wherein said first frustum supports said object, said second frustum rests on said object and said first angle is greater than said second angle.

11. Apparatus as claimed in claim 9, wherein said first frustum supports said object, said second frustum rests on said object and the altitude of said second frustum is greater than the altitude `of said first frustum.

12. Apparatus as claimed in claim 11, wherein the altitude of said second `frustum is approximately twice the altitude -of said first frustum.

References Cited UNITED STATES PATENTS 2,449,318 9/ 1948 Pitman et al. 219-10.81 2,548,093 4/ 1951 Blewitt 219-10.81 X 2,660,660 11/1953 Von Hauteville 219--10.81 2,668,226 2/ 1954 Bernard 34--1 JOHN I. CAMBY, Primary Examiner. 

